bennett



March 31, 1964 V w. F. BENNETT 3,127,252

FAIL-SAFE EXPLOSIVE GAS DISTRIBUTION SYSTEM Original Filed 001;. 28, 1959 4 Sheets-Sheet 1 WORK ZONE 65o 3c SO/QCE ZONE 490 2C [Q INVENTUQ LU. F'EE'NNE'T'T' March 31, 1964 w. F. BENNETT FAIL-SAFE EXPLOSIVE GAS DISTRIBUTION SYSTEM Original Filed Oct 28, 1959 4 Sheets-Sheet 2 I INVEN'TU LL/.F.' BENNETT March 31, 1964 w. F. BENNETT FAIL-SAFE EXPLOSIVE GAS DISTRIBUTION SYSTEM Original Filed Oct. 28, 1959 4 Sheets-Sheet 3 INVENT'UFZ LU. F BENNETT March 31, 1964 F. BENNETT 3,127,252

FAIL-SAFE EXPLOSIVE GAS DISTRIBUTION SYSTEM Original Filed Oct. 28, 1959 4 Sheets-Sheet 4 INVEN'TUQ LU. F.' BENNETT United States Patent 3,127,252 FAIL-SAFE EXPLUdli/E GAS DISTRiBUTION YTEM Wesiey F. Bennett, Reading, Pa, assigner to Western Electric Company, incorporated, New York, N.Y., a corporation of New York ()riginal application Get. 28, 1959, Ser. No. 849,413. Divided and this application Mar. 31, 19nd, Ser. No. 26,393

3 Claims. (Cl. 48-192) This application is a division of application Serial No. 849,413, filed October 28, 1959.

This invention relates to gas distribution systems and particularly to fail-safe distributing systems for explosive gases.

The necessity frequently arises to supply an explosive gas for a particular use. An example is the provision of a reducing gas atmosphere, usually hydrogen, to furnaces used in diffusing semiconductor materials. Since accessibility is an important feature in such devices, it is impractical to provide an absolutely gas-tight distribution system which would insure against the possibility of explosion.

It is an object of the invention to provide a fail-safe distributing system for explosive gases.

A further object of the invention is to provide a prac tical apparatus for quantity production of diffused semiconductor bodies.

One aspect of the invention comprises a fail-safe system for handling explosive gases, such as hydrogen, which may be used as a carrier gas. Valves are located at strategic points in the system for automatically turning Off the reducing gas supply and for turning On an inert purging gas supply when conditions at any one of the monitored points change beyond specified values. A secondary source of inert purging gas is provided in the event that the primary purging source fails. Automatic cut-over is provided in both stages.

This invention will be more clearly and fully understood from the exemplary embodiments when read with reference to the accompanying drawings in which:

FIG. 1 is a mechanical schematic view of a furnace suitable for use in one stage of the process according to one feature of the invention including a graph of the thermalprofile of the furnace.

FIG. 2 illustrates the gas distribution and purging system according to one embodiment of the invention.

FIG. 3 represents the electro-mechanical control circuit for FIG. 2.

FIG. 4 schematically represents the electrical power circuit for the embodiment of FIGS. 2 and 3.

It will be realized from the detailed description which follows that the semiconductor diffusion process used to illustrate the invention in one aspect may appropriately be termed a two-stage or two-tube diffusion process and apparatus. FIG. 1 shows the first of the so-called tubes or furnaces and, since it contains all of the elements necessary for the second tube, will also serve to illustrate the second stage of the invention. As pictured in FIG. 1, the first stage, shown generally as 10, consists of a furnace 11, having two sections 12 and 13 mounted in tandem. Thermocouple-controlled electric heating elements (not shown) are suitably placed so that the desired thermal profile is maintained. The controls are shown generally in FIG. 4 together with their possible power relation to other electric components. FIG. 1 also shows in graphic form the thermal profile to be maintained in the first stage of the process when an antimony impurity is diffused into a P-type germanium semiconductor body. Vertical lines extending between the furnace schematic and the graph related positions in the furnace and their associated temperature as represented on the graph. It will be noted that 3,127,252 Patented Mar. 31, 1964 the temperature rises from 25 degrees centigrade just outside the lefthand side of section 12 to 490 degrees centigrade at the central, or source zone, portion of section 12. This central portion of section 12 is the location of the solid antimony source during the vaporization stage for supplying the impurity vapor. The temperature then rises in the transition zone existing between sections 12 and 13 increasing to 650 degrees centigrade in the work zone, section 13. This temperature is maintained over the diffusion area of the furnace. A wide range of temperatures is available for inducing evaporation of the antimony, and selection of an appropriate value is dependent primarily on the concentration density desired and the method of heating the source and distributing the vapor. However, there is an upper limit to the diffusion temperature for P-type germanium. That limit, the conversion temperature at which P-type germanium is converted to N-type, is about 675 to 680 degrees centigrade. A zone 14 is provided at the righthand end of section 13 in which a gradual reduction of temperature to degrees centigrade occurs which provides a cooling zone upon withdrawal of the semiconductive wafer at the end of the first stage.

Gas input tubing 15 provides at different times hydrogen carrier gas and nitrogen purging gas which flow from left to right through tubing 18 and furnace sections, 12, 13 and 14. The gases, as well as unused antimony vapor, are removed at the exhaust hood 16. The hydrogen gas is burned off at the exit end of section 14 by means of a suitable burner (not shown) maintained with an external fuel gas.

A source of antimony is positioned in a small cup 17. The cup 17 is sealed in the furnace entry tubing 18 and may be moved into and out of section 12 by means of external magnet 19 cooperating with a magnetic member attached to the cup. This magnetic control arrangement permits a system of movement of the impurity source into and out of furnace section 12 without the trouble of packing glands and sliding joints, a particular advantage in a system utilizing hydrogen as a carrier gas. Element 20 functions to permit a boat 21 which holds the semiconductor bodies to be moved through the hydrogen burn-ofi flame into and out of section 13. A variable speed drive shown generally at 22 is positioned to control the move ment of carriage 28 and work boat 21 into and out of the furnace. The work boat drive arrangement consists of a constant speed reversible motor 23 directly connected to a variable speed drive 23' then to a gear reduction box upon whose output shaft is mounted a small capstan. By adjusting the variable drive, an infinite number of speeds may be obtained within its range. Further variations of drive speed are easily achievable by changing either the gear box or the diameter of the capstan. A cable 25 is wound around the capstan 24 then over a pulley 26 at the end of a track held on supporting frame 27. The carriage 2S rides on the track and supports a pushrod 29 which holds the member 20. Cable 25 passes through carriage 28 to a second pulley 39 to return it under the track support 22 where a simple weight 31 is fastened at the end of the cable with a pulley sheeve 32.

The second stage of the two-stage diffusion process utilizes a diffusion furnace similar in structure to that purging line 56.

unused source area of second stage providing it does not affect the profile of the work area. If identical profiles are used in the furnaces for both stages, the furnaces may be used interchangeably with an appropriate replacement of the first stage furnace muille with an antimonyfree second stage mufile.

To insure the safe and effective practice of the twostage method, a fail-safe system has been devised which insures the safe use of the explosive reducing gas on large scale production runs and minimizes contamination problems by providing for purging with an inert gas which contributes a non-explosive environment when dangerous conditions arise. This aspect of the invention will best be understood in connection with FIGS. 2, 3 and 4. The system envisages a source of explosive reducing gas, usually hydrogen, and two sources of inert purging gas, usually nitrogen. The first nitrogen source may be considered a general or house nitrogen supply and the second nitrogen source and an emergency auxiliary bottle supply.

In FIG. 2 the fail-safe system is shown generally as 40. The hydrogen input appears at 41, the house nitrogen input at 42, and the emergency nitrogen supply is shown at 43. For purposes of explanation, it will be assumed that the gases and the proper pressure are available at the input valves '44, 45 and 46, respectively, for hydrogen, house nitrogen, and emergency nitrogen. It will also be assumed that appropriate electrical power has been supplied for thesystem and that the three valves '44, 45 and 46 are open. The hydrogen can come through regulator 47 to the pneumatic valve 48. House nitrogen is present in regulator 49 and branch 50 prior to regulator 49. The bottle nitrogen can proceed to pneumatic valve 51. Pressure switches 52, 531 and 54 are set to open the electrical switches at a deficiency of gas pressure at the respective gas sources 41, 42 and 43. Closing pressure switch 53 places power On electro-pneumatic relay 55 which in turn supplies pressure to pneumatic valve 51. With pressure on pneumatic valve 51 house nitrogen flows from the house nitrogen line through 50 into purge line 56. With no pressure on pneumatic valve 1, the valve functions to connect the emergency nitrogen supply 43 to In addition, pneumatic valve 57 bloc-ks the application of house nitrogen pressure to pneumatic valve 48. With no pressure on pneumatic valve 48 the hydrogen line is closed.

A purging flow of house nitrogen can proceed along lower line 58 to the furnace input 59 (15 in FIG. 1). This is a manual safety system for purging the furnace at will. Manual control valve 60 controls the flow of nitrogen through line 58 to the furnace. Only a back-check valve 61 and a flame check 168 are between valve 60 and the furnace input tube 59.

In addition, nitrogen may flow up line 62 to valve 63 which is a manual three-way valve. Valve 63 in one setting allows-hydrogen to flow from left to right through it if all other operating requirements are satisfied. A second setting permits nitrogen to flow from the line 62 through the network shown generally at 64. The third position of valve 63 is shut off. It is desirable that valve 6 3 be set for nitrogen flow from 62 through hydrogen system 64 prior to the introduction of hydrogen in order to purge the hydrogen system and the furnace of contaminants before starting the process. Nitrogen can then flow through filter 65, flow gauge 66, and to valve 67 which controls the rate of gas flow. The gas then proceeds down through back-check 68 and the deoxidizing and drying element 69. The gas then flows through normally open valve 76, cold trap 71, and normally open valves 72, 73 and 74. Valve 73 is closed only in the reaotivating cycle of .deoxidizer 69 and valve 74 is an orifice valve set so that a maximum flow of approximately six liters per minute of hydrogen passes through the orifice at the three-pound pressure of the system. This 4 back pressure behind the orifice of valve 74 is suflicient to actuate pressure-sensitive switch 75.

In FIG. 2 an independent fuel gas system is shown at 76 which provides a fuel gas for burning off the gases supplied by system 46 as they exit from the furnace. A manually operated control valve 77 provides fuel gas to regulator 78. Thermal switch 79 is operable to open at a deficiency of fuel gas flame temperature.

Operation of the system will best be understood by referring to FIG. 3, in which reference numeral 80 denotes the main electrical control system operated in conjunction with the piping system of FIG. 2. Switches 8 1, 82, 83 and 8 4 are closed respectively by proper pressure on pressure elements 52, 53, 54 and 75 in FIG. 2. As a preliminary to starting the furnace the gas 'burn-oflf supply 76 of FIG. 2 is lit which closes switch 85 through temperature controlled switch 79 of FIG. 2. This completes the electrical power circuit to timer 86 which forces a time delay of about 25 minutes. During this period nitrogen flows through purging line 62 and network 64 into the furnace thoroughly purging the hydrogen system and the furnace before succeeding steps may be undertaken. The timer switch 86 closes after 25 minutes and places the main contactor 87 in condition for operation by manual starting buttons 88. The main contact 90 is then energized. This in turn energizes electro-pneumatic valve 57 (\FIGS. 2 and 3) which permits the house nitrogen pressure to actuate valve 48 which in turn permits the hydrogen to flow up valve 63 (FIG. 2). In addition, the relay 91, a double pole, double throw type, has interrupted the holding circuit or standby contactor 92 and has transferred the source of voltage of eleotro-pneurnatic valve 93 (FIGS. 2 and 3) to the main contactor 87. Eleotro-pneum atic valve 93- is the emergency purge line valve. When all of the above conditions have been met, the operator can switch from nitrogen purge of the hydrogen system to the hydrogen source by operating manual valve 63. The furnace is now in normal operation.

Emergency operation will now be described. If any one of the contacts 81, 82, 83, 84, or 85 are opened, due to a deficiency of pressure on elements 52, 53, 54 or 75 or flame failure at 79, power is removed from main con-tactor 87. This in turn opens up the main contact 90 which de-energizes self-venting electro-pneumatic valve 57, removing the nitrogen pressure from pneumatic valve 48 and closing off the hydrogen line (FIG. 2). In addition, relay 91 is also de-energized, and since the standby contactor, which is manually operated by push buttons 94, is de-energized, there is no source of voltage for emergency purge valve 93. As a result, valve 93 opens and emergency purging nitrogen flows through 93 and through hydrogen network 64 into the furnace. The increase in pressure to the right of 93 is sensed by pressure switch 111 (FIGS. 2 and 3) which blows a warning horn 1 12 and lights a warning light 96.

In the event that house nitrogen is the source of failure, pressure switch 53 (FIG. 2) opens, in turn opening switch 82 (FIG. 3) which deactivates 55 simultaneously relieving the pressure from both valves 48 and 51. Valve 48 closes the hydrogen line as before, and 51 transfers the purging line 56 to the emergency auxiliary nitrogen source 43. To re-establish operation of the furnace, the source of trouble must be located and corrected and the system must again progress through the six starting steps enumerated above to re-establish hydrogen flow. It will be noted that failure of the electric power supply, for example, by blowing of fuse 95, will also cause automatic purging, since 93 would be de-energized as well as the main contactor 87. In this event the warning horn 112 and light 96 will not operate until the power has been re-established. Purging, however, occurs nevertheless.

A series of lights connected in parallel across the input and output of main circuit 80 and between the different switching and push button elements 81 through 86 indicate the operating condition of the circuit at critical points, and, also, since the switches are operated by pressures in different parts of the gas distribution system, the pressure conditions in the gas system and the furnace. This warning light system comprises a simple trouble shooting aid since in the event of failure, other than a power failure, the lights between the power source and the failure point will remain lit and those between the point of failure and ground will go out.

The furnace is placed on standby by simultaneously manually deactivating the main contactor 87 and energizing standby contactor 92. This supplies power to and closes the purge line valve 93 and deactivates the electropneumatic valve 57 thereby closing valve 48 and shutting oif the hydrogen supply. Prior to placing the system on standby the operator has purged the system by manually switching valve 63 to the nitrogen supply.

FIG. 4 shows one possible arrangement for the power supply and the electrical relationship of the different elements of the system. The wires of a 220-volt, 3-phase, 60-cycle alternating current power suppy 100 are tapped through a transformer 101. Switching blocks 102 and 103 transmit the power to the gas panel system 104 (shown in more detail in FIG. 3) and electronic instrument controllers 105 as well as the deoxidizing and drying elements 110 of the gas distribution system. Additional lines fed through switches 106 control the furnace heat supply and the motor drive input 107 which moves the work boat into and out of the furnace.

It is to be understood that the above described arrangements and methods are simply illustrative of the principles of the invention. The gas distribution system including the entire electro-mechanical arrangement also can be modified to different embodiments while still remaining within the purview of the present invention. For example, additional bypaths may be provided across the hydrogen system to prevent the comparatively high nitrogen pressure from damaging the system in the event that the primary paths are blocked possibly by failure of a regulator or filter or like element. Those skilled in the art may also devise numerous other arrangements embodying the principles of the invention and falling within its spirit and scope.

What is claimed is:

1. An electro-mechanical control circuit for an explosive gas fail-safe distribution system comprising in combination a main network having a plurality of serially connected pressure-operated electrical switches, a plurality of pressure-sensitive means located in the gas distribution system and connected to respective ones of the switches, each switch being opened by a deficiency of pressure at the location of the associated pressure-sensitive means, timer means serially connected with the switches, the timer means having first and second parallel branches, the first of said branches providing a delayed time reaction for closing contacts in the second branch, starting means serially connected with the second branch, said starting means having third and fourth parallel branches, said third branch having manually operated means operable when said switches and said timer second branch are electrically interconnected to actuate contacts in said fourth branch, said fourth branch being serially connected with electro-mechanical means operable upon closure of the contacts in said fourth branch to open the gas distribution system from a source of the explosive gas and to maintain closed a valve for providing inert purging gas to the distribution system.

2. A control circuit according to claim 1 wherein said combination further comprises a standby network in parallel with said main network, said electro-mechanical means having switching means for transferring control of the explosive gas valve to said standby circuit when the electro-mechanical circuit is not operative, and said standby circuit having manually operative switching means serially connected with said valve control for providing inert gas to said distribution system.

3. An electro-mechanically controlled explosive gas distributing system which comprises in combination a first, a second, and a third gas source, the first source being of an explosive environmental gas and the second and third sources of an inert gas, first, second and third gas paths respectively between said first, second and third sources and a high temperature device for utilizing said gases, a first network portion of each of said paths being common to the other paths and a second network portion before said first portion comprising a common section of said second and third paths, a first valve means at the juncture of the second and third paths with said second portion, said first valve means being operable to connect the second source to the second portion when suificient pressure exists at the second source and to connect the third source to the second portion when insufiicient pressure exists at the second source, first, second and third pressure-sensitive means respectively at the outputs of each of said sources, a second valve means in said first path before said first portion operable by any one of said pressure-sensitive means to stop the supply of the explosive gas, a third valve means in said second portion operable by any one of said pressure-sensitive means to open the second portion and supply inert gas through the second and first network portions to the high temperature device, a fourth pressure-sensitive means responsive to the explosive and inert gas input pressures to the high temperature device, a fuel gas source for burning oif the explosive gas at the output end of the device, and a temperature-sensitive means responsive to the temperature of the burn-01f flame, the second valve and the third valve being responsive to a deficiency in pressure at either the fourth pressure-sensitive device or the temperature-sensitive means respectively to turn off the explosive environmental gas supply and to pass the inert gas supply through the system and into the high temperature device.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ELECTRO-MECHANICAL CONTROL CIRCUIT FOR AN EXPLOSIVE GAS FAIL-SAFE DISTRIBUTION SYSTEM COMPRISING IN COMBINATION A MAIN NETWORK HAVING A PLURALITY OF SERIALLY CONNECTION PRESSURE-OPERATED ELECTRICAL SWITCHES, A PLURALITY OF PRESSURE-SENSITIVE MEANS LOCATED IN THE GAS DISTRIBUTION SYSTEM AND CONNECTED TO RESPECTIVE ONES OF THE SWITCHES, EACH SWITCH BEING OPENED BY A DEFICIENCY OF PRESSURE AT THE LOCATION OF THE ASSOCIATED PRESSURE-SENSITIVE MEANS, TIMER MEANS SERIALLY CONNECTED WITH THE SWITCHES, THE TIMER MEANS HAVING FIRST AND SECOND PARALLEL BRANCHES, THE FIRST OF SAID BRANCHES PROVIDING A DELAYED TIME REACTION FOR CLOSING CONTACTS IN THE SECOND BRANCH, STARTING MEANS SERIALLY CONNECTED WITH THE SECOND BRANCH, SAID STARTING MEANS HAVING THIRD AND FOURTH PARALLEL BRANCHES, SAID THIRD BRANCH HAVING MANUALLY OPERATED MEANS OPERABLE WHEN SAID SWITCHES AND SAID TIMER SECOND BRANCH ARE ELECTRICALLY INTERCONNECTED TO ACTUATE CONTACTS IN SAID FOURTH BRANCH, SAID FOURTH BRANCH BEING SERIALLY CONNECTED WITH ELECTRO-MECHANICAL MEANS OPERABLE UPON CLOSURE OF THE CONTACTS IN SAID FOURTH BRANCH TO OPEN THE GAS DISTRIBUTION SYSTEM FROM A SOURCE OF THE EXPLOSIVE GAS AND TO MAINTAIN CLOSED A VALVE FOR PROVIDING INERT PURGING GAS TO THE DISTRIBUTION SYSTEM. 